I think an ideal and inexpensive training aid would be to hang a sheet or a net near where you're going to throw. Weight the bottom so it hangs nicely but it would provide a "straight line" that wouldn't injure you if you hit it. Pull the disc back as close to the sheet as you can, pull through maintaining the same distance, and try to release the disc as your wrist "turns the corner" straight down the line the sheet points on.